Tuesday, 6 May 2014

Pheno 2014 Liveblog: Day One Session 2

More coffee, new session.  Now it's theory, so that might also help me stay awake.

11:00 am: Electroweak symmetry breaking after the Higgs, Martin Schmaltz

"We've discovered the Higgs, now what?  We don't really know."

The Higgs Lagrangian mass (μ ~ 89 GeV) is quite small compared to the scale we are now probing, the TeV scale.  Similarly the quartic coupling (λ ~ 0.13) is smallish compared to 1.  Is it generated at loops?  (Composite pNGB models).

The most important thing from the CMS/ATLAS Higgs plots is that ATLAS likes to make more complicated plots!

At this point, naive theorist says CMS + ATLAS points to all SM compatible.  2HDMs require heavy Higgses > 300 GeV, which corresponds to a ratio of mass squareds of ~ 0.1.  Hence approximate as a single Higgs theory.  Higgs EFT: dimension 6 operators.  I had a friend who was at a conference in Spain a couple of weeks ago, where apparently they had two weeks of talks on nothing but Higgs dimension 6 couplings.  It was, he said, great motivation to start new projects on anything else!

These all point to no new physics (below at least a TeV), which we could have said even before the LHC turned on (LEP measurements of precision observables), plus modern measurements of Higgs couplings only making it worse.

This brings in the elephant in the room, naturalness.  The Higgs mass is sensitive to any new scale in the theory; quantum corrections give mH ~ Λ with Λ the scale of non-SM physics.  There are essentially four answers:
  1. Symmetry arguments protect the Higgs mass (SUSY, pNGB).  This is the standard argument, but has the problem that we would expect other evidence for these models (partners of the top and/or gauge bosons).  This is the usual problem that we haven't seen them.
  2. No new scales: UV physics is scale invariant.  If the largest relevant scale is the top mass, the Higgs is natural.  This is Strumia's finite naturalness argument.  The problem is that we do have scales:
    • The Planck scale; maybe gravity is different, but why?
    • The hypercharge Landau pole.  If we can ignore gravity, this would seem to be a genuine problem.
    • The transition scale: if we introduce new physics to avoid the Landau pole, we must introduce a correction to the Higgs mass (1308.0025).
  3. Anthropics: deferred for Nima's talk on Wednesday.  I'm still not convinced that life requires a light Higgs/weak scale, though.  Neither is Martin, it seems.
  4. Something else.  The comparison here is to the UV catastrophe of 19th century physics.  It looked like physics was close to being worked out, but it turned out that QM would radically revolutionise our understanding of reality.  What is our revolution?  Of course, some people would argue that the answer to that question is point 3 on this list.
11:35 am: PDF's for Standard Model physics and beyond, Joey Huston

The crux is that pdfs are a major theoretical uncertainty on any physics at a hadron collider.  So we have detailed comparisons of all(?) different pdf fits, to quantify those uncertainties.  It shows that the uncertainties aren't that bad; in most cases they are good.  This is all at NNLO, which seems to be the current state of the art.

Unfortunately, it does still remain the case that the large-x region (the "discovery region") is still quite poorly known.  Specifically, for production of 2 TeV states, we are looking at ~30% theoretical uncertainties.

There is also a problem at lower masses.  The gg pdf is now the largest theoretical uncertainty in the Higgs cross section!  A systematic disagreement with the different fits is a few percent.

Another repeated theme in this talk is the importance of HERA data, to the point that even new LHC data may well not overcome it except at the highest energy and x.  HERA2 will also be very important.  I guess this is luminosity + precision measurements.

A final interesting point is the development of "meta-pdfs", which not only serve as averages over the different pdfs but includes the relevant theoretical error.

12:10 pm: Bridging theory and experiments, Valentin Hirschi

Looking at these slides I draw too conclusions:
  1. This is a talk on Monte Carlos.
  2. It has way too many slides.
The reason for that is that we have a lot of basic material covered.  That's okay at Pheno, I guess; for a lot of the attendees this might still be quite new.  But I'm not seeing much that really jumps out at me.

Merging/Matching: at tree level, this is "simple".  Use a scale above which you generate only in the hard process, and below which only in the parton shower.  At NLO it is hard because we need to include zero-momentum real emission to cancel IR divergences in loops.  But this problem has been solved (in multiple ways).  Rather than have a sharp switch from hard process to parton shower point, have a smoother transition that sometimes uses Parton shower and sometimes uses hard process.  Finite by construction.

Something interesting and worth noting!  A new program, Galileo, that takes symmetry + field content and writes most general Lagrangian.  I'm downloading it now.  Or at least I would, if I could find it.

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